The Tax Office said it had begun to receive calls from businesses with Macs that were having trouble accessing its Business Portal, which is used by hundreds of thousands of small businesses across Australia to lodge their business activity statements (BAS) that most are required to send to it at least every three months to pay their taxes.

"Recent changes made by Apple to block programs using Oracle Java have affected Mac users when they seek to ... access online services using their AUSkey," the website states. "AUSkey advise that the issue is due to Apple recently removing the Java plug-in from their latest update of the operating system."

It's understood a number of other Tax Office online systems use AUSkey and are also affected by the block. As well as the Business Portal requiring Java, the Tax Agent Portal requires AUSkey too.

Fairfax Media contacted the Tax Office after businesses complained that they were unable to lodge their activity statements.

Oracle has since released a version of Java that isn't blocked, but because most versions don't automatically install updates it has left many users confused as to what to do.

Security expert Chris Gatford, of HackLabs, praised the block around the time it was announced because he believed Java was a "dying technology".

From a security perspective, he said Java had been used by malware writers for quite some time to exploit computer systems.

It was a "highly recommended practice" to turn it off, Mr Gatford said.

The majority of Mac users, who use Mac OS 10.7 and 8, will have to manually download the latest version of Java, released over a week ago, by following instructions on Apple's website and going into System Preferences to install it. Others running Mac OS 10.6 and under which already had Java on their Mac before the block was applied would have been prompted to update Java for it to work again.

With an agent's log-in, a criminal gains access to an agent's existing client records, which has information on how much each client earns.

Some tax agents warned a criminal could also potentially access other Australian taxpayers' information via the portal if they knew four pieces of information about a person. They said a criminal could potentially lodge a fake tax return on behalf of someone else and claim the money themselves.

The Tax Office has denied that all Australians' tax information was put at risk, however has not disputed the agents' claims. It refuses to respond to further questions about the breach.